The demand for an electrochemical element, such as a lithium-ion secondary battery, an electric double layer capacitor and a hybrid capacitor including a lithium-ion capacitor, has rapidly been expanded because of its characteristics such as small size, lightweight and high energy density as well as capability of repeated charge-discharge. The lithium-ion secondary battery is used in the fields of mobile phones, laptop PC and the like, because of its relatively large energy density. Also, the electric double layer capacitor is used as a small-sized electric source for memory backup for PC and the like, because of its capability of rapid charge-discharge. Furthermore, the electric double layer capacitor can be expected for the application as a large-size electric source for an electric vehicle. Also, the hybrid capacitor, which combines both advantages of the lithium-ion secondary battery and the electric double layer capacitor, has drawn attention due to its high energy density and power density. Further improvements such as lowered resistance, increased capacity and improved mechanical properties are required in these electrochemical elements as the applications are expanded and developed.
The hybrid capacitor has a polarizable electrode for its positive electrode and a non-polarizable electrode for its negative electrode, and uses an organic electrolyte, which allows increasing both operating voltage and energy density. For the hybrid capacitor, it is proposed to use a material, obtained by preliminarily storing lithium-ion in a material able to store and release lithium-ion by a chemical method or electrochemical method, for the negative electrode (e.g. Patent Documents 1 and 2).
As a method for making the negative electrode preliminarily carry lithium in the case of a large-scale cell such as an electric source for automobile, it is proposed that lithium derived from the negative electrode is carried by electrochemical contact with lithium source arranged to face the negative electrode or the positive electrode in an organic electrolytic battery where each of a positive electrode collector and a negative electrode collector has pores penetrating front and back surfaces thereof, and a negative electrode active material is capable of reversibly carrying lithium (e.g. Patent Document 3). In Patent Document 3, the collector is provided with pore penetrating front and back surfaces, and electrode active material layers are formed on both sides of the perforated collector (hereinafter the collector having penetrating pore may be referred to as “perforated collector”). Due to such a constitution, the capacitance can be improved and also, lithium-ion enables to move between both sides of the electrode without being blocked by the collector, so that not only the negative electrode arranged near the lithium source but also the negative electrode arranged away from the lithium source can electrochemically carry lithium ion via through pores even in an electric storage device having a cell constitution with a large number of stacked layers. Also, discharge and charge proceeds smoothly because lithium-ion freely moves between respective electrodes via the through pores.
The electrode active material layer can be formed by coating slurry of electrode composition including an electrode active material, conducting material and binder on a collector followed by drying. Particularly, there is proposed a twin-blade method, wherein a pair of dies are arranged on both sides of a path of the collector running in vertical direction for the purpose of simultaneously forming electrode active material layers on the both sides of the collector, and a pair of blades are provided above the pair of dies to control a coating thickness by scraping the slurry ejected from the dies with the blades. However, for the perforated collector having through pores, it is difficult to coat the slurry with uniform thickness, so that the thickness of the electrode active material layer and the active material amount in the obtained electrode may vary to cause variation in electrode performance. Also, since the slurry is coated on both sides of the collector in this method, two dies are always required and furthermore, two sets of tanks for coating material, supply pump, filter, pipe and the like are respectively needed, resulting in complexed equipment and increased cost. Also, it is necessary to strictly adjust clearance of the two dies, ejecting amount of the slurry, clearance of die lip portion, etc. for controlling the coating thickness and surface condition of the electrode. Furthermore, the slurry may be transferred on a rotating roller when the perforated collector such as punching metal and expanded metal is fed to a common horizontal coating machine such as Comma Coater, so that it is difficult to coat the slurry uniformly on the collector.
As a method for forming the electrode active material layer on the perforated collector with an uniform thickness, for example, Patent Document 4 discloses a method wherein electrode material is formed into a sheet and agglutinated to the collector at one time by providing electrode material to a pair of press rolls by using a metering feeder and also providing the collector between the press rolls.
Also, Patent Document 5 proposes a method for forming the electrode active material layer on the collector by contacting slurry coated on a base material with a perforated collector for unification, followed by drying the slurry and peeling the base material. In this method, the slurry layer where the base material is layered is dried, so that it is hard to uniformly evaporate the solvent of the slurry. Therefore, in Patent Document 5, a porous base material is used for the base material for uniformly evaporating the solvent and obtaining a uniform thickness of the dried electrode active material.    [Patent Document 1] The Japanese Unexamined Patent Publication H03-233860,    [Patent Document 2] The Japanese Unexamined Patent Publication H05-325965,    [Patent Document 3] International Publication No. 98/33227 (which corresponds to U.S. Pat. No. 6,461,769),    [Patent Document 4] The Japanese Unexamined Patent Publication 2007-5747    [Patent Document 5] The Japanese Unexamined Patent Publication 2008-41971